1. Field of the Invention
This invention relates generally to optical sensors and more particularly to a staring infrared sensor having a large Field Of Regard (FOR) while also having the small Instantaneous Field Of View (IFOV) needed for improved dynamic range at higher resolutions.
2. Description of the Prior Art
Infrared sensors with relatively large Fields Of Regard (FORs) are needed for viewing a large portion of the horizon. Such sensors that also have a relatively small Instantaneous Field Of View (IFOV) are generally classified as Infrared Search and Track (IRST) sensors. With appropriate signal processing, these IRST sensors can detect and track targets at ranges otherwise too large to permit resolution of the target. The target detection range is highly dependent on the signal to noise ratio at the IRST sensor, which depends on the available IFOV and integration time as well as other factors. IRST sensors are known in the art to include the scanning sensor, the step-stare sensor and the staring sensor.
The scanning sensor has a small IFOV that is mechanically scanned through the desired Field Of Regard (FOR). A well-known disadvantage of the scanning sensor is the requirement for a large, heavy and expensive electromechanical scanning apparatus. Another well-known disadvantage is the necessarily short sensor integration time, which reduces the sensitivity of the sensor.
The step-stare sensor has a small IFOV that is mechanically scanned through the desired FOR, but uses a mirror that rotates in the opposite direction in steps to momentarily keep the image stationary. This effectively increases the available integration time. A well-known disadvantage of the step-stare sensor is the complexity and cost of the requisite mechanically-scanned mirror.
The staring infrared sensor has a relatively small FOR because of the narrow FOV lens needed to keep the IFOV to an acceptably small value. A well-known disadvantage of the staring sensor is the number of individual sensors required to cover the desired FOR. The disadvantage of using an array of many staring sensors is the increased weight, volume and cost.
In view of these deficiencies, there is accordingly a well-known need for an inexpensive infrared sensor that provides a large Field Of Regard (FOR) and a small IFOV for long-range surveillance applications. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the problem of viewing a large Field Of Regard (FOR) while also integrating over a small IFOV by subdividing the FOR into a plurality of internal optical paths without the use of mechanically-movable parts.
It is a purpose of this invention to provide a low cost staring infrared imaging sensor capable of viewing a large portion of the horizon, and detecting small dim targets at long standoff ranges. It is a feature of this invention that each of the plurality of internal optical paths may be further subdivided by a plurality of steerable micro-mirrors to further reduce the IFOV of the invention.
In one aspect, the invention is a staring infrared imaging sensor including a primary lens disposed to accept infrared radiation and to project therefrom a Field Of Regard (FOR) image onto a primary image plane, a plurality of primary mirrors disposed at the primary image plane each for reflecting a corresponding portion of the FOR image along a corresponding one of a plurality of optical paths, a secondary lens disposed within each optical path to focus the corresponding FOR image portion onto a corresponding secondary image plane within the corresponding optical path, a secondary mirror disposed at the corresponding secondary image plane within each optical path to reflect the corresponding FOR image portion along one of a corresponding plurality of preselected directions, an image detector disposed at a tertiary image plane for generating an electronic signal representing an image projected onto the tertiary image plane, and a tertiary lens disposed within each optical path to focus the corresponding FOR image portion onto the tertiary image plane.
In a preferred embodiment, each secondary mirror includes a plurality of micro-mirrors each movable from one to another of a plurality of positions, whereby an instantaneous field of view (IFOV) image within the corresponding FOR image portion can be redirected from one to another of the preselected directions.
In another aspect, the invention is an infrared detection system including a staring infrared imaging sensor having a primary lens disposed to accept infrared radiation and to project therefrom a Field Of Regard (FOR) image onto a primary image plane, a plurality of primary mirrors disposed at the primary image plane each for reflecting a corresponding portion of the FOR image along a corresponding one of a plurality of optical paths, a secondary lens disposed within each optical path to focus the corresponding FOR image portion onto a corresponding secondary image plane within the corresponding optical path, a secondary mirror disposed at the corresponding secondary image plane within each optical path to reflect the corresponding FOR image portion along one of a corresponding plurality of preselected directions, an image detector disposed at a tertiary image plane for generating an electronic signal representing an image projected onto the tertiary image plane, and a tertiary lens disposed within each optical path to focus the corresponding FOR image portion onto the tertiary image plane, and a controller coupled to the secondary mirrors for apportioning the amount of time during which the corresponding FOR image portion is directed along any one of the corresponding plurality of preselected directions.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.